Children you have studied the general
structure of Eukaryotic cells. Though all the eukaryotic cells have the same
basic structure. There are some fundamental differences between animal andplant cells. These are shown in the Table.
Cell wall is present outside the plasma membrane of
bacteria, algae, fungi and all higher plants. But, its structure and chemical
composition is different in different groups. Carbohydrates are the major
components of the cell wall. The type of carbohydrate present in the cell wall
of each of these groups in different.
In bacteria, the cell wall is made up of carbohydrates
other than cellulose. In other words, cellulose is absent in the cell walls of
bacteria. In higher plants, cell wall is made up of cellulose, hemicelluloses,
pectin (as calcium or magnesium salts) lignin.
Cell wall gives mechanical strength to the cell.
Without the cell walls, plants cannot support their long and tall body.
Moreover, when the plants are watered, cells absorb water and swell. It the
cell wall is not present, the swollen cell will burst and die. As the cell wall
is tough and hard, it prevents excess swelling of the cell. As a result, the
cell becomes slightly stretched and turgid.
When there is no water, the cell shrinks and no
pressure is exerted on the cell wall. The plant looses the stiffness and
droops. This is called wilting.
Adjacent cells in a plant are connected through long
tube like structures called plasmodesmata. These tubes penetrate through the
cell walls.
Vacuoles are fluid filled bag like structures
surrounded by a membrane. These are present in the cytoplasm of plant cells. In
a newly formed plant cell, the vacuoles are small. As the cell becomes old,
these vacuoles fuse to form a single large vacuole that fills the entire volume
of the cell.
In such
cells, cytoplasm is in the form of a thin rim around vacuole and the nucleus is
pushed to one side. The membrane surrounding the vacuoles is called Tonoplast
and it allows only certain substances to pass through. The fluid in the
vacuoles is called vacuolar sap and contains a variety of substances such as
carbohydrates, amino acids, proteins, pigments and waste material.
Plastids are the organelles which are responsible
for the colour of the plant cell. Various colours you observe in plants,
especially in flowers and leaves are due to the presence of plastids in the
cells of these parts. Depending on their structure, colour and function,
plastids are of three types.
Leucoplast (Leuco=white) :- these are white in
colour and are usually seen in plant parts which are not exposed to sun light.
They are present in the cells of seeds, tubers, rocs…… and rhizomes. They are
of different shapes – oval, rod like, spherical or filamentous. They store food
materials.
Chloroplasts :- These are green in colour. They
occur in plant parts exposed to sun light. They contain chlorophyll, in
addition to the DNA present in the nucleus and ribosomes in the cytoplasm a
small amount of DNA and ribosomes are also present in the chloroplasts.
Major function of chloroplasts is to trap solar
energy and convert this to chemical bond energy. As you have studied,
chlorophyll plays a major role in photosynthesis.
These contain fat soluble pigments which may be red,
orange or yellow. They are present in fruits and flowers and are responsible
for their colours.
All the three plastids have the capacity to change
from one to another. For example, young tomatoes are white in colour – as they
mature they turn to green and then to red in colour. You might have seen green
chillies becoming red and several flowers and fruits changing their colour.
Cell wall and plastids are present only in plant
cells and absent in animal cells. Centrioles are present in animal cells and
are absent in plant cells.
In animal cells, two centrioles are present usually
on the top of the nucleus. Together, they are called centrosome. Centrioles are
short, stout cylindrical structures and participate in the cell division you will learn more about them in next lesson.
Points to remember:
Cell wall plastids and vacuoles are present in plant
cells and are absent in animals cells. Animal cells have centrioles and plant
cells do not have centrioles.
Cell wall is also present in bacteria but is
structurally and chemically different from that of plant cell wall.
Cell wall gives mechanical strength to the cell and
also helps the cells when they are turgid. Plasodesmata connect adjacent cells.
Vacuoles occupy most of the volume of the cell and
are filled with various substances. The membrane surrounding the vacuole is
called tonoplast.
Leucoplasts store food materials while chloroplasts
help in the preparation of food materials by participating photosysnthesis.
chromoplsts give colour to flowers and fruits.
Choloroplasts have chlorophyll which required for photosynthesis
. they also have DNA and robisomes inside them. Centrioles participate in cell division.
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